Method for acquiring system frame number by terminal, terminal, and mobile communication system

ABSTRACT

A System Frame Number (SFN) acquisition method is provided. The System Frame Number (SFN) acquisition method of a terminal according to the present invention includes receiving a first message for adding a secondary cell of a secondary base station from a primary cell of a primary base station, receiving a Master Information Block (MIB) broadcast in the secondary cell, and acquiring a SFN information for the secondary cell from the MIB, and applying the SFN information to at least one cell of the secondary base station.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation application of prior application Ser.No. 14/760,386, filed on Jul. 10, 2015, which issued as U.S. Pat. No.10,306,604 on May 28, 2019; which is the U.S. National Stage applicationunder 35 U.S.C § 371 of an International application numberPCT/KR2014/010412, filed on Nov. 3, 2014; and which is based on andclaimed priority of a Korean patent application number 10-2013-0132572,filed on Nov. 1, 2013, in the Korean Intellectual Property Office, thedisclosure of each of which is incorporated by reference herein in itsentirety.

TECHNICAL FIELD

The present invention relates to a method and apparatus for operating asmall cell effectively in a LTE-based small cell environment and, inparticular, to a method, terminal, and mobile communication system foracquiring System Frame Number (SFN).

BACKGROUND ART

Mobile communication systems were developed to provide the subscriberswith voice communication services on the move. With the rapid advance oftechnologies, the mobile communication systems have evolved to supporthigh speed data communication services beyond the early voice-orientedservices. However, the limited resource and user requirements for higherspeed services in the current mobile communication system spur theevolution to more advanced mobile communication systems.

As one of the next-generation mobile communication systems to meet suchrequirements, standardization for a Long Term Evolution Advanced (LTE-A)system is underway in the 3rd Generation Partnership Project (3GPP).LTE-A is one of the high speed packet-based communication technologiessupporting data rate higher than that of the current mobilecommunication technology under the objective to complete thestandardization in late 2010.

With the evolvement of the 3GPP standard, many discussions are beingconducted for optimizing the radio network in addition to the effort forincreasing data rate. In mobile communication systems, small cellscharacterized by small service areas are frequently used to increasesystem throughput and remove coverage holes. However, the small celldeployment causes considerable problems in view of supporting mobilitysuch as handover failure. Nevertheless, researches about systemparameter application or operation mechanism suitable for small sizeservice areas are very scarce.

DISCLOSURE OF INVENTION Technical Problem

The present invention aims to provide a System Frame Number (SFN)acquisition method of a terminal capable of using different SFNs betweenSecondary Cell Group (SCG) and Master Cell Group (MCG), method, andmobile communication system.

Solution to Problem

In accordance with an aspect of the present invention, a System FrameNumber (SFN) acquisition method of a terminal is provided. The SFNacquisition method includes receiving a first message for adding asecondary cell of a secondary base station from a primary cell of aprimary base station, receiving a Master Information Block (MIB)broadcast in the secondary cell, and acquiring a SFN information for thesecondary cell from the MIB, and applying the SFN information to atleast one cell of the secondary base station.

In accordance with another aspect of the present invention, a method fora base station to provide a System Frame Number (SFN) is provided. Themethod includes determining to add a secondary cell of the base stationin cooperation with the other base station connected to the terminalthrough a primary cell and transmitting a Maser Information Block (MIB)including SFN information to be applied to at least one of the basestation through the secondary cell.

In accordance with another aspect of the present invention, a method fora base station to provide a System Frame Number (SFN) is provided. Themethod includes connecting a primary cell of the base station to aterminal, determining to add a secondary cell of another base station tothe terminal, and transmitting a first message including systeminformation of the secondary cell except for SFN to be applied to atleast one cell of the other base station, wherein the SFN to be appliedto at least one cell of the other base station is broadcast in thesecondary cell.

In accordance with another aspect of the present invention, a terminalis provided. The terminal includes a transceiver which transmits andreceives signals to and from a primary and secondary base stations and acontrol unit which controls the transceiver to receive a first messagefor adding a secondary cell of the secondary base station from a primarycell of the primary base station and to receive a Master InformationBlock (MIB) broadcast in the secondary cell, acquires a SFN informationfor the secondary cell from the MIB, and applies the SFN information toat least one cell of the secondary base station.

In accordance with another aspect of the present invention, a basestation is provided. The base station includes a transceiver whichtransmits and receives signals to and from a terminal and another basestation and a control unit which determines to add a secondary cell ofthe base station in cooperation with the other base station connected tothe terminal through a primary cell and control the transceiver totransmit a Maser Information Block (MIB) including SFN information to beapplied to at least one of the base station through the secondary cell.

In accordance with still another aspect of the present invention, a basestation is provided. The base station includes a transceiver whichtransmits and receives signals to and from a terminal and another basestation and a control unit which connects a primary cell of the basestation to a terminal, determines to add a secondary cell of anotherbase station to the terminal, and controls the transceiver to transmit afirst message including system information of the secondary cell exceptfor SFN to be applied to at least one cell of the other base station,wherein the SFN to be applied to at least one cell of the other basestation is broadcast in the secondary cell.

Advantageous Effects of Invention

The SFN acquisition method, terminal, and mobile communication system ofthe present invention are advantageous in terms of applying differentSFNs to the Secondary Cell Group (SCG) and Master Cell Group (MCG) byallowing a cell belonging to the SCG to provide SFN. Also, the SFNacquisition method, terminal, and mobile communication system of thepresent invention are advantageous in terms of applying different SFNsto the SCG and MCG by allowing a cell belonging to the SCG to providethe SFN in such a way of allocating the SFN and SFN offset value to theterminal.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram for explaining a small cell deployment scheme;

FIG. 2 is a flowchart illustrating a UE operation in the case where SFNinformation to be applied to an SCG is transmitted through a specificserving cell belonging to the SCG;

FIG. 3 is a flowchart illustrating a UE operation in the case where anoffset value for use in calculating an SFN to be applied to an SCG;

FIG. 4 is a diagram illustrating the concept of the RRC diversitytechnique;

FIG. 5 is a signal flow diagram illustrating a basic procedure ofapplying the RRC diversity;

FIG. 6 is a signal flow diagram illustrating a procedure for the UE toactivate the RRC diversity according to a predetermined condition;

FIG. 7 is a signal flow diagram illustrating a procedure for the eNB toactivate the RRC diversity according to predetermined conditions;

FIG. 8 is a diagram for explaining a situation in which a UE fails toestablish a connection to any system but just continues performingredirection;

FIG. 9 is a diagram for explaining a method of solving the problem ofrepetitive occurrence of UMTS-LTE redirection;

FIG. 10 is a signal flow diagram illustrating a method of solving therepetitive UMTS-LTE redirection problem;

FIG. 11 is a block diagram illustrating a configuration of a UEaccording to an embodiment of the present invention;

FIG. 12 is a block diagram illustrating a configuration of an eNBaccording to an embodiment of the present invention;

FIG. 13 is a signal flow diagram illustrating a procedure of providing aUE with an SFN offset value;

FIG. 14 is a diagram for explaining a method of deriving an SFN offsetvalue; and

FIG. 15 is a signal flow diagram illustrating a procedure for a UE toprovide the information for use in deriving the SFN offset informationfor itself.

MODE FOR THE INVENTION

Exemplary embodiments of the present invention are described withreference to the accompanying drawings in detail. The same referencenumbers are used throughout the drawings to refer to the same or likeparts. Detailed description of well-known functions and structuresincorporated herein may be omitted to avoid obscuring the subject matterof the present invention.

Although the embodiments of the present invention are directed toAdvanced E-UTRA (or LTE-A) supporting carrier aggregation, it will beunderstood to those skilled in the art that the subject matter of thepresent invention can be applied to other communication systems havingthe similar technical background and channel format, with slightmodification, without departing from the spirit and scope of the presentinvention. For example, the subject matter of the present invention canbe applied to the multicarrier HSPA supporting carrier aggregation.

The specification and drawings are to be regarded in an illustrativerather than a restrictive sense in order to help understand the presentinvention. It is obvious to those skilled in the art that variousmodifications and changes can be made thereto without departing from thebroader spirit and scope of the invention.

The present invention relates to a method and apparatus for operatingsmall cells effectively in the LTE-based small cell environment. Thepresent invention proposes a method of providing a terminal (UserEquipment; UE) with a System Frame Number (SFN) information of the smallcell in the case where the macro and small cells use different SFNs.Also, the present invention proposes a method of applying RRC diversityto the UE operating in a dual connectivity mode for connecting to themacro and small cells simultaneously. Prior to the explanation of thepresent invention, a description is made of deployment of cells havingsmall service areas. In the following description, the cell having arelatively small service area is referred to as small cell. In thefollowing embodiment, the cell of which the radius is in the range from5 to 200 m is referred to as small cell.

In the following description, the term ‘SFN information’ is used in themeaning of including at least one of an SFN and information indicatingthe SFN. Also, the term ‘SFN offset value’ is used in the meaning ofincluding at least one of an SFN offset indicator and an SFN offset.

FIG. 1 is a diagram for explaining a small cell deployment scheme.

Referring to FIG. 1, the first deployment scheme is to deploy small cellevolved Node Bs (eNBs) 105 sparsely in the service area of the macrocell eNB 100. Here, the sparse deployment may have a meaning ofdeploying the small cells sparsely so as not to be overlapped with eachother. Such deployment is useful to cover hot spot areas where trafficsare concentrated or remove coverage holes. Depending on whether thesmall cell uses a frequency identical with or different from that of themacro cell, the movement of a UE from the macro cell to the small cellor vice versa causes intra-frequency or inter-frequency handover (HO).As another deployment scheme, it can be considered to deploy a pluralityof small cells densely. The dense deployment may have a meaning ofdeploying the small cells densely so as to be overlapped with eachother. The UE may connect to the macro and small cells simultaneously toreceive data. This is referred to as Dual Connectivity. Particularly, aset of the serving cells under the control of a small cell eNB (smalleNB) is referred to as a Secondary Cell Group (SCG), and a set of theserving cells under the control of a macro cell eNB (macro eNB) isreferred to as a Master Cell Group (MCG).

Embodiment 1

This embodiment proposes two methods of providing the SFN information tothe SCG when different SFNs are applied to the MCG and SCG. In thelegacy LTE Rel-10 Carrier Aggregation (CA), the same SFN(System FrameNumber) is applied to the Primary Cell (PCell) and Secondary Cell(SCell). The CA technique is characterized in that a service is providedthrough a plurality of serving cells unlike the conventional techniquein which a service is provided through one serving cell. Here, the cellplaying the same role as the legacy serving cell is the PCell. The eNBcan receive feedback information from the UE through a Physical UplinkControl Channel (PUCCH) and perform the operations related to handoverand Radio Link Monitoring/Radio Link Failure (RLM/RLF) in the PCell.Also, the UE acquires essential information from the System Information(SI) broadcast by the eNB. Examples of the information may include SFN,frequency bandwidth, cell ID, access barring information, and idle modecell measurement configuration information. Meanwhile, the servingcell(s) configured in addition to the PCell is (are) the SCell(s). Thesystem informations of the SCells are provided through dedicatedsignaling, and the SCells do not participate in the operations relatedto handover and RLM/RLF. In the system information, the SFN is set to avalue in the range from 0 to 1023 in order to number the radio frames.That is, as time goes, the SFN increases by 1 every radio frame. InRel-10 CA, the same SFN is applied to both the PCell and SCell. ThePCell broadcasts the SFN information through MIB as part of the systeminformation, and the UE applies the SFN information to both the PCelland SCell. Accordingly, the system information of the SCell which isprovided through dedicated signaling has no SFN information. The SFN isused for various functionalities such as System Information Block (SIB)scheduling, Discontinuous Reception (DRX) configuration, measurementgap, enhanced Inter-Cell Interference Coordination (eICIC), In DeviceCoexistence (IDC), and Relay Node (RN).

In the small cell environment, the macro and small cells eNBs coexistseparately. Accordingly, it may be difficult to implement the procedureof applying the same SFN to the macro and small cells due to thesynchronization difficulty. In the case that the deployed eNBs aremanufactured by different manufacturers, it may be more difficult toimplement the procedure of applying the same SFN to the macro and smallcells. However, it is preferred to apply the same SFN to the servingcells at least belonging to the MCG or the same SCG. The presentinvention proposes two methods of providing the SCG with SFN informationwhich is different from that of MCG when different SFNs are applied tothe MCG and SCG.

The first method is characterized by designating a serving cell whichbroadcasts the SFN information in the SCG such that the UE acquires theSFN information applied to the SCG. In the present invention, theserving cell providing the SFN information is referred to as primarySecondary Cell (pSCell). The pSCell is characterized in that it hasPUCCH and is capable of receiving the feedback information of theserving cells of the SCG.

FIG. 2 is a flowchart illustrating a UE operation in the case where SFNinformation to be applied to an SCG is transmitted through a specificserving cell belonging to the SCG.

Referring to FIG. 2, the UE adds an SCell at step 200. The UE determineswhether the SCell belongs to the MCG or the SCG at step 205. If theSCell belongs to the MCG, the UE applies the MIB of the SCell which hasbeen received through a Radio Resource Control (RRC) message at step210. The UE applies the SIB of the SCell which has been received throughthe RRC message at step 215. The UE applies the SFN information includedin the MIB broadcast through the PCell at step 220. The UE applies theSFN information to perform PUCCH transmission and/or DRX operation atstep 225. If the SCell belongs to the SCG at step 205, the UE determineswhether the SCell is the pSCell at step 230. If so, the UE applies theMIB information including no SFN which has been received through the RRCmessage at step 235. Here, the UE may receive the RRC message throughthe PCell or SCell of the MCG. The RRC message includes the systeminformation to be applied to the SCell as well as the SCellconfiguration information. The UE applies the SIB of the SCell which hasbeen received through the RRC message at step 240. The UE decodes thePBCH (channel carrying the MIB) broadcast through the pSCell to acquirethe SFN at step 245. The UE applies the SFN information to perform PUCCHtransmission and/or DRX operation at step 250. If the SCell is not thepSCell at step 230, the UE applies the MIB of the SCell which has beenreceived through the RRC message at step 255. Here, the UE may receivethe RRC message through the pSCell of the SCG. The UE applies the SIB ofthe SCell which has been received through the RRC message at step 260.The UE applies the SFN information of the pSCell at step 265. At step265, the UE performs PUCCH transmission and/or DRX operation by applyingthe SFN information.

The second method is characterized by allowing the eNB to provide the UEwith an offset value indicating the difference between the SFNs appliedto the MCG and SCG in order for the UE to calculate SFN for the SCGthrough dedicated signaling. In this way, the UE can calculate the SFNto be applied to the SCG.

FIG. 3 is a flowchart illustrating a UE operation in the case where anoffset value for use in calculating an SFN to be applied to an SCG.

Referring to FIG. 3, the UE adds an SCell at step 300. The UE determineswhether the SCell belongs to the MCG or an SCG at step 305. If the SCellbelongs to the MCG, the UE applies the MIB of the SCell which has beenreceived through an RRC message at step 310. The UE applies the SIB ofthe SCell which has been received through the RRC message at step 315.The UE applies the SFN information included in the MIB broadcast throughthe PCell at step 320. The UE performs PUCCH transmission and/or DRXoperation by applying the SFN information at step 325. If the SCellbelongs to an SCG at step 305, the UE applies the MIB informationreceived through the RRC message at step 330. The RRC message includesan SFN offset value indicating the difference between the SFNs to beapplied to the MCG and SCG. The RRC message includes the systeminformation to be applied to the SCell as well as the SCellconfiguration information. The UE applies the SIB of the SCell which hasbeen received through the RRC message at step 335. The UE calculates theSFN value to be applied to the SCG using the SFN information broadcastthrough the PCell of the MCG and the SFN offset received through the RRCmessage at step 340. The SFN value applied to the current SCG is definedas ‘SFN value to be applied to MCG+SFN offset’. The unit of SFN offsetvalue may be l0 ms which is equal to the length of a radio frame. The UEperforms PUCCH transmission and/or DRX operation by applying the SFNinformation acquired through the above calculation at step 345.

Embodiment 2

This embodiment is directed to an RRC diversity technique. The RRCdiversity is a technique of improving the successful receptionprobability of an RRC message by transmitting/receiving the same RRCmessage repeatedly from/to the multiple eNBs to/from the UE. The RRCdiversity can be applied in downlink (DL) and uplink (UL). In the DL RRCdiversity, the UE receives the same RRC message from a plurality ofeNBs. In the UL RRC diversity, the UE transmits the same RRC message toa plurality of eNBs. This may increase successful transmissionprobability of the RRC message at the cell edge.

FIG. 4 is a diagram illustrating the concept of the RRC diversitytechnique.

Referring to FIG. 4, the UE 400 receives the same RRC message from thetwo eNBs, i.e. the Master eNB (MeNB) 405 and the Secondary eNB (SeNB)410, simultaneously. The RRC message addressed to the UE is exchangedthrough the Xn backhaul between the MeNB and SeNB. The two eNBs mayoperate on the same frequency or different frequencies. The UE may notreceive the RRC message because the signal strength of the MeNB is notstrong enough. If the SeNB transmits the same RRC message to the UE too,the successful reception probability may improve. Typically, it isexpected that the RRC diversity is triggered only when a certain levelof diversity gain is predicted but not always. It is preferred to applythe RRC diversity only when a predetermined level of diversity gain ispredicted instead of applying the RRC diversity by taking notice of UEtransmission power constraint, low uplink load in comparison to downlinkload, and UE complexity for processing multiple RRC message. The presentinvention proposes an enhanced method of applying the RRC diversity.

FIG. 5 is a signal flow diagram illustrating a basic procedure ofapplying the RRC diversity.

Referring to FIG. 5, the UE sends the MeNB 505 the UE capabilityindicating whether the UE 500 supports RRC diversity at step 515. TheMeNB 505 configures or triggers the RRC diversity by transmitting apredetermined RRC message, e.g. RRC Connection Reconfiguration, at step520. At steps 525 and 530, the UE 500, the MeNB 505, and the SeNB 510exchange the same RRC message. The simplest method of applying the RRCdiversity is to apply the RRC diversity always after configuration.However, if a specific link is very good, it may be possible to transmitthe RRC message through the corresponding link with the high successfultransmission probability. In this case, the RRC diversity may cause moreradio resource waste and more unnecessary UE and/or eNB operations thandiversity gain. Accordingly, it is more effective to apply the RRCdiversity only when the RRC diversity gain is expected. The presentinvention proposes a method of applying the RRC diversity selectively inuplink. Particularly, the present invention proposes two methods thatare selectively applied depending on whether the RRC diversity isactivated by the UE or the eNB. Although the description has beendirected to the uplink, the present invention is applicable to thedownlink in a similar way.

FIG. 6 is a signal flow diagram illustrating a procedure for the UE toactivate the RRC diversity according to a predetermined condition.

Referring to FIG. 6, the UE 600 sends the MeNB 605 a UE capabilitymessage indicating whether the UE 600 supports the RRC diversity at step615. The MeNB 605 configures the RRC diversity to the UE 600 using apredetermined RRC message at step 620. Although the RRC diversity hasbeen configured, it is not activated immediately until a predeterminedcondition is fulfilled. At steps 625 and 630, the UE sends an RRCmessage to the MeNB 605 and the SeNB 610.

Alternatively, the UE may transmit the RRC message via the best one ofthe two links to the MeNB and SeNB. For example, if the received signalquality of one of macro and small cell links is greater than apredetermined threshold X, the UE transmits the RRC message to the linkon which the signal quality is better than the other. If it isdetermined that the predetermined conditions for obtaining the diversitygain is fulfilled at step 635, the UE activates the RRC diversity andduplicates the RRC message on the RRC or PDCP layer at step 640 andtransmits the messages to both the eNBs at steps 645 and 650. In thestate that the RRC diversity is activated, the various conditions fordetermining whether the diversity gain is expected. For example, thereare conditions 1 and 2 as follows.

-   -   Condition 1: If the difference of the received signal qualities        of two links (macro and small cells) is equal to or greater than        a predetermined threshold Y and the received signal qualities of        both the two links are lower than a predetermined threshold Z,        the RRC diversity is activated.    -   Condition 2: If the received signal qualities of both of the two        links are lower than the threshold Z, the RRC diversity is        activated.

Here, the received signal quality may be one of Reference SignalReceived Power (RSRP), Reference Signal Received Quality (RSRQ), BlockError Rate (BLER), and Channel Status Indication (CSI). Theaforementioned thresholds Y and Z may be the values which arepredetermined fixedly or the values which the eNB configures andtransmits to the UE. If the above conditions are not fulfilled, the UEstops the ongoing RRC diversity.

FIG. 7 is a signal flow diagram illustrating a procedure for the eNB toactivate the RRC diversity according to predetermined conditions.

Referring to FIG. 7, the UE 700 sends the MeNB 750 a UE capabilitymessage indicating whether the UE 700 supports the RRC diversity at step715. The MeNB 705 configures the RRC diversity to the UE 700 using apredetermined RRC message at step 720. The RRC diversity configurationinformation may include RRC diversity activation conditions and relatedthreshold values. Although the RRC diversity has been configured, it isnot activated immediately until a predetermined condition is fulfilled.At steps 725 and 730, the UE sends an RRC message to the MeNB 705 andthe SeNB 710. Alternatively, the UE may transmit the RRC message via thebest one of the two links to the MeNB and SeNB. For example, if thereceived signal quality of one of macro and small cell links is greaterthan a predetermined threshold X, the UE transmits the RRC message tothe link on which the signal quality is better than the other. The UEsends the MeNB 705 (or SeNB 710) the cell measurement information atstep 735. The eNB determines whether the conditions for obtaining thediversity gain with the activation of the RRC diversity is fulfilledbased on the measurement information received from the UE at step 740.If it is determined to activate the RRC diversity, the MeNB 705 sendsthe UE 700 a predetermined RRC message including an indicator indicatingactivation of the RRC diversity at step 745. This indicator is set to 0for indicating activation of the RRC diversity or 1 for indicatingdeactivation of the RRC diversity. The UE deactivates the RRC diversityand duplicates the RRC message on the RRC or PDCP layer at step 750 andtransmits the duplicated message to both of the two eNBs at steps 755and 760. In the state that the RRC diversity is activated, the variousconditions for determining whether the diversity gain is expected. Theconditions have been described above. If a predetermined condition isfulfilled, the UE reports the cell measurement information to the eNBperiodically, and the eNB determines whether to control the UE to stayin the RRC diversity mode based on the measurement information. If it isdetermined that the RRC diversity gives no gain any more, the eNB sendsthe UE the RRC message including the indicator indicating deactivationof the RRC diversity.

Embodiment 3

If a predetermined cause occurs, the Universal Mobile TelecommunicationSystem (UMTS) and LTE systems reject the connection request of the UEand perform redirection to facilitate connection to the counterpartsystem. If the connection request is rejected, the UE attemptsconnection to the counterpart system based on the redirectioninformation. Depending on the case, however, the UE may fail toestablish a connection to any system and just repeat redirection. Thepresent invention proposes a method to resolve this non-preferredsituation.

Prior to the explanation of the present invention, the problems to besolved are described. FIG. 8 is a diagram for explaining a situation inwhich a UE fails to establish a connection to any system but justcontinues performing redirection.

Referring to FIG. 8, a UE 805 attempts connection establishment with aUMTS base station 800. For this purpose, the UE 805 sends the basestation 800 an RRC CONNECTION REQUEST message at step 810. If the basestation 800 rejects the connection request for a predetermined reasonsuch as network congestion, it sends the UE 805 an RRC CONNECTION REJECTmessage at step 815. This RRC message may include redirectioninformation optionally. This information indicates another frequency orsystem to which the UE has to attempt connection establishment. The UEof which the connection request has been rejected attempts connection toanother frequency or system at step 820. For example, the UE may beredirected to the LTE system. The UE attempts connection to the LTE basestation 830 and sends the MME of the LTE system an RRC CONNECTION SETUPCOMPLETE message carrying a Tracking Area Update (TAU) REQUEST messageat step 825. The UE 805 notifies that it has move to the LTE networkusing the TAU REQUEST message. However, the LTE system may not supportthe UE 805 for a certain reason. For example, the LTE system may have nocontract for providing services for the roaming UE 805. If the LTEsystem does not support the UE 805 for such a reason, the MME sends theUE 805 a TAU REJECT message over the DL INFORMATION TRANSMFER message atstep 835. Also, the LTE base station 830 sends the UE 805 an RRCCONNECTION RELEASE message to terminate the RRC connected mode at step840. At this time, the UMTS system is likely to keep still providing theUE with the very good channel. Accordingly, the UE 805 attemptsconnection to the UMTS base station 800 again at step 845. However, theconnection request is rejected and thus the UE 805 repeats theabove-described operations so as to stay in the state unconnected to anysystem. In order to escape from this situation, it is required torestrict attempting connection to the LTE system which does not providethe service to the UE 805. The present invention proposes a method tosolve the above problem without extra UE operations such as frequencymeasurement and SI reception and decoding.

FIG. 9 is a diagram for explaining a method of solving the problem ofrepetitive occurrence of UMTS-LTE redirection.

Referring to FIG. 9, a UE 900 which has a contract for a service in aUMTS network but not in an LTE network moves to the LTE networkaccording to the redirection information and sends an MME an ATTACH (orTAU) request message at step 905. The MME checks that there is nocontract for serving the UE 900 and then sends the UE 900 an ATTACHREJECT message. The ATTACH REJECT message includes an MME causeindicating the rejection reason. In the above case, a MME cause #15 isincluded along with a new indicator. The MME cause #15 denotes that theroaming UE is not allowed in the corresponding Tracking Area (TA), andthe UE which has received this message excludes the frequency indicatedby the new indicator in the cell reselection process for 300 seconds. Inthis case, the new indicator includes the frequency information (F1) ofthe LTE network to which the UE 900 is attempting connection. In themethod according to the present invention, the UE which has received theMME cause #15 and the new indicator is not served in all TAs but only ina specific TA. Accordingly, it is regarded that the access attempts toall TAs on the indicated frequency are blocked. For reference, table 1shows IE EMM causes that are indicated with total 2 bytes respectively.

TABLE 1 Cause value (octet 2) Bits 8 7 6 5 4 3 2 1 0 0 0 0 0 0 1 0 IMSIunknown in HSS 0 0 0 0 0 0 1 1 Illegal UE 0 0 0 0 0 1 0 1 IMEI notaccepted 0 0 0 0 0 1 1 0 Illegal ME 0 0 0 0 0 1 1 1 EPS services notallowed 0 0 0 0 1 0 0 0 EPS services and non-EPS services not allowed 00 0 0 1 0 0 1 UE identity cannot be derived by the network 0 0 0 0 1 0 10 Implicitly detached 0 0 0 0 1 0 1 1 PLMN not allowed 0 0 0 0 1 1 0 0Tracking Area not allowed 0 0 0 0 1 1 0 1 Roaming not allowed in thistracking area 0 0 0 0 1 1 1 0 EPS services not allowed in this PLMN 0 00 0 1 1 1 1 No Suitable Cells In tracking area 0 0 0 1 0 0 0 0 MSCtemporarily not reachable 0 0 0 1 0 0 0 1 Network failure 0 0 0 1 0 0 10 CS domain not available 0 0 0 1 0 0 1 1 ESM failure 0 0 0 1 0 1 0 0MAC failure 0 0 0 1 0 1 0 1 Synch failure 0 0 0 1 0 1 1 0 Congestion 0 00 1 0 1 1 1 UE security capabilities mismatch 0 0 0 1 1 0 0 0 Securitymode rejected, unspecified 0 0 0 1 1 0 0 1 Not authorized for this CSG 00 0 1 1 0 1 0 Non-EPS authentication unacceptable 0 0 1 0 0 0 1 1Requested service option not authorized 0 0 1 0 0 1 1 1 CS servicetemporarily not available 0 0 1 0 1 0 0 0 No EPS bearer contextactivated 0 1 0 1 1 1 1 1 Semantically incorrect message 0 1 1 0 0 0 0 0Invalid mandatory information 0 1 1 0 0 0 0 1 Message type non-existentor not implemented 0 1 1 0 0 0 1 0 Message type not compatible with theprotocol state 0 1 1 0 0 0 1 1 Information element non-existent or notimplemented 0 1 1 0 0 1 0 0 Conditional IE error 0 1 1 0 0 1 0 1 Messagenot compatible with the protocol state 0 1 1 0 1 1 1 1 Protocol error,unspecified

At this time, the UMTS system is likely to be still providing the UEwith very good channel. Accordingly, the UE attempts connection to theUNITS base station again at step 915. However, the connection requestmust be rejected again. The UE may consider the frequency F1 as a cellreselection candidate after 300 seconds at step 925. For cellreselection, the UE performs cell measurement and decodes the SystemInformation received from the LTE cell on the frequency F1. However,since the access to all of the TAs of the cell is barred, the UE cannotcamp on any cell. As a consequence, the UE does not attempt ATTACH ontoF1 but has to try cell measurement and SI acquisition at every 300seconds. This causes a significant shortcoming of increasing the powerconsumption of the UE. In order to solve this problem, the presentinvention proposes two methods. In the first method, when the access ofthe UE to all TAs is barred, the UE rules out the correspondingfrequency in the cell reselection process during the timer period longerthan 300 or continuously. In the second method, if the access of the UEto all TAs belonging to a predetermined PLMN is barred, the UE rules outall LTE frequencies belonging to the PLMN in the cell reselectionprocess. Since all of the LTE frequencies are excluded, the UE does notperform cell selection and SI acquisition continuously regardless of thepredetermined time of 300 seconds.

FIG. 10 is a signal flow diagram illustrating a method of solving therepetitive UMTS-LTE redirection problem.

Referring to FIG. 10, the UE 1000 redirected from the UMTS networkattempts access to the eNB 1005 by sending the RRC Connection Requestmessage at step 1015. The eNB 1005 sends the UE 1000 an RRC ConnectionSetup message at step 1020. The UE 1000 sends the eNB 1005 an RRCConnection Setup Complete message containing the TAU Request messageaddressed to the MME 1010 at step 1025. In the EMM-DEREGISTERED state,the MME 1010 rejects the UE 1000 at step 1035. The MME 1010 sends the UE1000 a TAU REJECT message including the EMM cause #15 indicating therejection reason and a new indicator at step 1040. The TAU REJECTmessage is delivered to the UE 1000 through a DL information transfer onthe Uu interface at step 1045. The UE stops performing the cellmeasurement and SI acquisition operations which are scheduled to be doneat every 300 according to the UE operation proposed in the presentinvention at step 1050.

FIG. 11 is a block diagram illustrating a configuration of a UEaccording to an embodiment of the present invention.

Referring to FIG. 11, the UE includes a transceiver 1100, amultiplexer/demultiplexer 1105, an upper layer entity 1110, a controlmessage processor 1115, and a controller 1120. The transceiver mayinclude a reception unit for receiving signals from an eNB and atransmitter for transmitting signals to the eNB. In the case oftransmitting control signals and/or data to the eNB, the UE multiplexesthe controls signals and/or data by means of themultiplexer/demultiplexer 1105 and transmits the multiplexed signal bymeans of the transceiver 1100 under the control of the controller 1120.In the case of receiving signals, the UE receives a physical signal bymeans of the transceiver 1100, demultiplexes the received signal bymeans of the multiplexer/demultiplexer 1105, and delivers thedemultiplexed information to the upper layer entity 1110 and/or controlmessage processor 1115, under the control of the controller 1120.

FIG. 12 is a block diagram illustrating a configuration of an eNBaccording to an embodiment of the present invention.

Referring to FIG. 12, the eNB includes a transceiver 1205, a controller1210, a multiplexer/demultiplexer 1220, a control message processor1235, upper layer entities 1225 and 1230, and a scheduler 1215. Thetransceiver 1205 may include a transmitter for transmitting signals to aUE and a reception unit for receiving signals from the UE. Thetransceiver 1205 transmits data and control signals through a downlinkcarrier and receives data and predetermined control signals through anuplink carrier. In the case that multiple carriers are configured, thetransceiver 1205 transmits/receives the data and control signals throughthe multiple carriers. The multiplexer/demultiplexer 1220 multiplexesthe data generated by the upper layer entities 1225 and 1230 and thecontrol message processor 1235 or demultiplexes the data received by thetransceiver 1205, the demultiplexed data being delivered to the upperlayer entities 1225 and 1230, the control message processor 1235, andthe controller 1210 appropriately.

FIG. 13 is a signal flow diagram illustrating a procedure of providing aUE with an SFN offset value.

Referring to FIG. 13, the UE 1300 receives the MIB information broadcastby the MeNB 1305 at step 1315. The MIB includes the SFN information tobe applied to the MeNB 1305 (hereinafter, referred to as MeNB SFN).

The MeNB 1305 may request the SeNB 1310 for the SFN information to beapplied to the SeNB 1310 (hereinafter, referred to as SeNB SFN).According to some embodiments, the SeNB 1310 may be the eNB of an SCellbelonging to an SCG. In order to provide the UE 1300 with the SFN offsetinformation, the MeNB 1305 has to receive the SeNB SFN from the SeNB1310. Step 1320 may be performed for this purpose.

The SeNB 1310 may provide the MeNB 1305 with the SeNB SFN informationperiodically or when a predetermined event occurs. The SeNB SFNinformation may be included in the SeNB configuration information whichis transmitted when the MeNB 1305 configures an SCell of the SeNB firstto the UE 1300. The SeNB 1310 may send the MeNB 1305 the SeNB SFNinformation through the X2 interface. The X2 interface is the interfacefor use in exchanging information between eNBs in the LTE system. TheSeNB SFN information includes an absolute time value for the case of thepredefined SFN value or a predetermined SFN value and the absolute timevalue corresponding thereto. For example, the predefined SFN value maybe the last SFN=0. Here, the absolute time value denotes the absolutetime at the start, middle, or end of the radio frame indicated by thepredefined SFN. The small cell eNB 1310 may provide the MeNB 1305 with apredetermined SFN value and the absolute time value of the start,middle, or end of the radio frame corresponding to the SFN value. Inthis case, the SeNB 1310 has to signal a predetermined SFN value inaddition.

The MeNB 1305 derives the SFN offset value using the SeNB SFNinformation provided by the SeNB 1310 at step 1330. Detailed descriptionof the offset value derivation procedure is made with reference to FIG.14. The MeNB 1305 compares the absolute time values corresponding to theMeNB SFN and SeNB SFN to derive the SFN offset value.

The MeNB 1305 sends the UE 1300 the SFN offset value using apredetermined RRC message at step 1335. At this time, the InformationElement (IE) indicating the SFN offset value included in the RRC messagemay be equal to or shorter than 10 bits.

The UE 1300 adds the SFN offset value to the current MeNB SFN value toderive the SeNB SFN value at step 1340. According to some embodiments,step 1340 may include steps 340 and 345 of FIG. 3. According to someembodiments, the RRC message of step 330 of FIG. 3 may be the messagetransmitted at step 1335.

FIG. 14 is a diagram for explaining a method of deriving an SFN offsetvalue.

The embodiment of FIG. 14 is characterized in that an SeNB 1400 includesthe absolute time of the start, middle, or end of the radio frameindicated by a predetermined SeNB SFN in the information provided to anMeNB 1405. The MeNB 1405 derives the SFN offset value using thedifference between the absolute time of a predetermined SeNB SFN and theabsolute time corresponding to the MeNB SFN having the same value. TheSeNB 1400 and the MeNB 1405 may be unsynchronized as much as denoted byreference number 1410. The SeNB 1400 records the absolute time 2004 msof the start time point of the frame which the last SeNB SFN=0 indicatesas denoted by reference number 1420 and transmits the recorded time tothe MeNB 1405. Although this embodiment is directed to the case wherethe start time point is considered, it is possible to consider theabsolute time of the middle or end time point of the frame. Since theradio frame 1415 has the fixed length of 10 ms, it is possible to derivethe SFN offset regardless of time point of the frame of which theabsolute value is considered. The transmission timing is provided to theMeNB 1405 in response to the request from the MeNB 1405, when apredetermined event such as SeNB SCell addition occurs, or periodically.The MeNB 1405 checks the absolute time of 2040 ms 1425 of the start timepoint of the frame indicated by the last MeNB SFN=0. The MeNB 1405calculates the difference between the absolute time of 2004 ms providedby the SeNB 1410 and the time 2040 and derives the SFN offset valueconsidering the SFN unit (10 ms) as denoted by reference number 1430. Asa consequence, the MeNB SFN and SeNB SFN have an SFN difference as muchas 3. The SFN offset value is transmitted to the UE through an RRCmessage. In this case, if the current MeNB SFN is 3, the SeNB SFNbecomes 6.

FIG. 15 is a signal flow diagram illustrating a procedure for a UE toprovide the information for use in deriving the SFN offset informationfor itself.

Steps 1515 to 1525 of FIG. 15 correspond to steps 1315 to 1325 of FIG.13. Thus detailed descriptions of steps 1515 to 1525 are omitted herein.After then, however, the MeNB 1505 forwards the information receivedfrom the SeNB 1510 to the UE 1500 at step 1530 other than that itderives the SFN offset value and transmits the SFN offset value to theUE 1500.

At step 1535, the UE 1500 performs the operation which the MeNB 1505performs at step 1330 of FIG. 13. The UE 1500 has to know the absolutetime corresponding to the MeNB SFN having the same value as the absolutetime of the received SeNB SFN. This information may be acquiredconsidering the reception time of the MIB including the MeNB SFNinformation. That is, if the absolute time when the MIB including theinformation indicating SFN=0 is acquired is 0 ms, the absolute time ofSFN=3 is 30 ms. The UE 1500 adds the SFN offset value to the currentMeNB SFN value to derive the SeNB SFN value at step 1540. The UE 1500may perform PUCCH transmission and/or DRX operation by applying theacquired SeNB SFN information.

According to some embodiment, the UE may perform steps 300 to 345 ofFIG. 3. The RRC message received at step 330 of FIG. 3 may include theSeNB SFN information transmitted at step 1530 instead of the SFN offsetvalue, and steps 340 of FIG. 3 may be substituted by steps 1535 and1540.

The SeNB SFN information may be provided by an O&M server as well as theSeNB 1510. Typically, the O&M server may store various informationsabout eNBs. Such informations may include SeNB SFN timing information.

It will be understood that each block of the flowchart illustrationsand/or block diagrams, and combinations of blocks in the flowchartillustrations and/or block diagrams, can be implemented by computerprogram instructions. These computer program instructions may beprovided to a processor of a general purpose computer, special purposecomputer, or other programmable data processing apparatus to produce amachine, such that the instructions, which execute via the processor ofthe computer or other programmable data processing apparatus, createmeans for implementing the functions/acts specified in the flowchartand/or block diagram block or blocks. These computer programinstructions may also be stored in a non-transitory computer-readablememory that can direct a computer or other programmable data processingapparatus to function in a particular manner, such that the instructionsstored in the non-transitory computer-readable memory produce an articleof manufacture including instruction means which implement thefunction/act specified in the flowchart and/or block diagram block orblocks. The computer program instructions may also be loaded onto acomputer or other programmable data processing apparatus to cause aseries of operational steps to be performed on the computer or otherprogrammable apparatus to produce a computer implemented process suchthat the instructions which execute on the computer or otherprogrammable apparatus provide steps for implementing the functions/actsspecified in the flowchart and/or block diagram block or blocks.

Furthermore, the respective block diagrams may illustrate parts ofmodules, segments or codes including at least one or more executableinstructions for performing specific logic function(s). Moreover, itshould be noted that the functions of the blocks may be performed indifferent order in several modifications. For example, two successiveblocks may be performed substantially at the same time, or may beperformed in reverse order according to functions thereof.

According to various embodiments of the present disclosure, the term“module”, means, but is not limited to, a software or hardwarecomponent, such as a Field Programmable Gate Array (FPGA) or ApplicationSpecific Integrated Circuit (ASIC), which performs certain tasks. Amodule may advantageously be configured to reside on the addressablestorage medium and configured to be executed on one or more processors.Thus, a module may include, by way of example, components, such assoftware components, object-oriented software components, classcomponents and task components, processes, functions, attributes,procedures, subroutines, segments of program code, drivers, firmware,microcode, circuitry, data, databases, data structures, tables, arrays,and variables. The functionality provided for the components and modulesmay be combined into fewer components and modules or further separatedinto additional components and modules. In addition, the components andmodules may be implemented such that they execute one or more CPUs in adevice or a secure multimedia card.

It is to be appreciated that those skilled in the art can change ormodify the embodiments without departing from the technical concept ofthis invention. Accordingly, it should be understood thatabove-described embodiments are essentially for illustrative purposeonly but not in any way for restriction thereto. Thus the scope of theinvention should be determined by the appended claims and their legalequivalents rather than the specification, and various alterations andmodifications within the definition and scope of the claims are includedin the claims.

Although preferred embodiments of the invention have been describedusing specific terms, the specification and drawings are to be regardedin an illustrative rather than a restrictive sense in order to helpunderstand the present invention. It is obvious to those skilled in theart that various modifications and changes can be made thereto withoutdeparting from the broader spirit and scope of the invention.

1. A method by a terminal in a wireless communication system, the methodcomprising: receiving, from a first base station, a radio resourcecontrol (RRC) message for adding at least one secondary cell of a secondbase station; receiving, on a primary secondary cell (PSCell) of thesecond base station, master information block (MIB) or physicalbroadcast channel (PBCH); identifying a system frame number (SFN) basedon the MIB or the PBCH; and applying the identified SFN to communicatewith the at least one secondary cell of the second base station, whereinthe RRC message includes information for the at least one secondary cellexcept for the SFN, and wherein the PSCell is configured with a physicaluplink control channel (PUCCH) resource.
 2. The method of claim 1,wherein a master cell group (MCG) is controlled by the first basestation, and wherein a secondary cell group (SCG) is controlled by thesecond base station.
 3. The method of claim 1, wherein a timing of theSFN associated with the at least one secondary cell of the second basestation is different from a time of an SFN associated with at least onecell of the first base station.
 4. A method by a second base station ina wireless communication system, the method comprising: determining toadd at least one secondary cell including a primary secondary cell(PSCell) having a physical uplink control channel (PUCCH) resource, ofthe second base station; and transmitting, on the PSCell to a terminal,master information block (MIB) or physical broadcast channel (PBCH), theMIB or the PBCH includes a system frame number (SFN) which is applied tocommunicate with the at least one secondary cell of the second basestation, wherein a radio resource control (RRC) message which istransmitted from a first base station to the terminal for adding the atleast one secondary cell to the terminal includes information for the atleast one secondary cell except for the SFN.
 5. The method of claim 4,wherein a secondary cell group (SCG) is controlled by the second basestation, and wherein a master cell group (MCG) is controlled by thefirst base station.
 6. The method of claim 4, wherein a timing of theSFN associated with the at least one secondary cell of the second basestation is different from a time of an SFN associated with at least onecell of the first base station.
 7. A method by a first base station in awireless communication system, the method comprising: determining to addat least one secondary cell of a second base station to a terminal; andtransmitting, to the terminal, a radio resource control (RRC) messagefor adding the at least one secondary cell of the second base station,wherein the RRC message includes information for the at least onesecondary cell except for a system frame number (SFN), wherein the SFNwhich is applied to the at least one secondary cell of the second basestation is transmitted to the terminal on master information block (MIB)or physical broadcast channel (PBCH) of a primary secondary cell(PSCell) of the second base station, and wherein the PSCell isconfigured with a physical uplink control channel (PUCCH) resource. 8.The method of claim 7, wherein a master cell group (MCG) is controlledby the first base station, and wherein a secondary cell group (SCG) iscontrolled by the second base station.
 9. The method of claim 7, whereina timing of the SFN associated with the at least one secondary cell ofthe second base station is different from a time of an SFN associatedwith at least one cell of the first base station.
 10. A terminal in awireless communication system, the terminal comprising: a transceiver;and a controller coupled with the transceiver and configured to controlto: receive, from a first base station, a radio resource control (RRC)message for adding at least one secondary cell of a second base station,receive, on a primary secondary cell (PSCell) of the second basestation, master information block (MIB) or physical broadcast channel(PBCH), identify a system frame number (SFN) based on the MIB or thePBCH, and apply the identified SFN to communicate with the at least onesecondary cell of the second base station, wherein the RRC messageincludes information for the at least one secondary cell except for theSFN, and wherein the PSCell is configured with a physical uplink controlchannel (PUCCH) resource.
 11. The terminal of claim 10, wherein a mastercell group (MCG) is controlled by the first base station, and wherein asecondary cell group (SCG) is controlled by the second base station. 12.The terminal of claim 10, wherein a timing of the SFN associated withthe at least one secondary cell of the second base station is differentfrom a time of an SFN associated with at least one cell of the firstbase station.
 13. A second base station in a wireless communicationsystem, the second base station comprising: a transceiver; and acontroller coupled with the transceiver and configured to control to:determine to add at least one secondary cell including a primarysecondary cell (PSCell) having a physical uplink control channel (PUCCH)resource, of the second base station, and transmit, on the PSCell to aterminal, master information block (MIB) or physical broadcast channel(PBCH), the MIB or the PBCH includes a system frame number (SFN) whichis applied to communicate with the at least one secondary cell of thesecond base station, wherein a radio resource control (RRC) messagewhich is transmitted from a first base station to the terminal foradding the at least one secondary cell to the terminal includesinformation for the at least one secondary cell except for the SFN. 14.The second base station of claim 13, wherein a secondary cell group(SCG) is controlled by the second base station, and wherein a mastercell group (MCG) is controlled by the first base station.
 15. The secondbase station of claim 13, wherein a timing of the SFN associated withthe at least one secondary cell of the second base station is differentfrom a time of an SFN associated with at least one cell of the firstbase station.
 16. A first base station in a wireless communicationsystem, the base station comprising: a transceiver; and a controllercoupled with the transceiver and configured to control to: determine toadd at least one secondary cell of a second base station to a terminal,and transmit, to the terminal, a radio resource control (RRC) messagefor adding the at least one secondary cell of the second base station,wherein the RRC message includes information for the at least onesecondary cell except for a system frame number (SFN), wherein the SFNwhich is applied to the at least one secondary cell of the second basestation is transmitted to the terminal on master information block (MIB)or physical broadcast channel (PBCH) of a primary secondary cell(PSCell) of the second base station, and wherein the PSCell isconfigured with a physical uplink control channel (PUCCH) resource. 17.The first base station of claim 16, wherein a master cell group (MCG) iscontrolled by the first base station, and a secondary cell group (SCG)is controlled by the second base station.
 18. The first base station ofclaim 16, wherein a timing of the SFN associated with the at least onesecondary cell of the second base station is different from a time of anSFN associated with at least one cell of the first base station.